1. Field of the Invention
This invention relates to a process for producing polypropylene having excellent see-through property which comprises preliminarily polymerizing 3-methylbutene-1 using a specific catalyst system, and then polymerizing propylene or copolymerizing propylene and an alpha-olefin in the presence of the catalyst used in the preliminary polymerization. The invention also pertains to a stretched polypropylene film having excellent see-through property with very little losses of transparency and rigidity which is obtained by stretching polypropylene containing a specified amount of 3-methylbutene-1 polymer in at least one direction.
2. Description of the Prior Art
Many propylene polymers and propylene/alphaolefin copolymers having a high stereospecificity index, and many processes for their production are known.
It is also known that the propylene polymers and propylene/alpha-olefin copolymers can be produced with a high catalytic efficiency.
Attempts have been made to improve the see-through property or transparency of propylene polymers and propylene/alpha-olefin copolymers.
For example, Japanese Laid-Open Patent Publication No. 80329/1983 discloses a specific method of dispersing sodium benzoate as a nucleating agent in polypropylene in order to improve its transparency. Japanese Patent Publication No. 12460/1980 discloses a method in which 1.3,2.4-di(methylbenzylidine)sorbitol is incorporated in crystalline polypropylene to increase its transparency
Japanese Laid-Open Patent Publication No. 32430/1970 (corresponding to British Patent 1,104,665) discloses a process for producing a solid polymer moulding composition comprising at least 55% by weight propylene monomer units, monomer units of a second component, which is a linear 1-olefin having from 4 to 18 carbon atoms, in an amount of up to 35% by weight and having dispersed throughout the composition polymer units of a third component, which is an alpha-olefin whereof the homopolymer has a crystalline melting point above 180.degree. C., present in an amount of up to 10% by weight, which comprises bringing propylene, the second component and the third component into contact with a stereospecific catalyst composed of a compound of a transition metal of Groups IV to VI activated by an organometallic compound or a metal hydride. This patent document states that the resulting composition has improved transparency over polypropylene resin, and 3-methyl-1-pentene, 4,4-dimethyl-1-pentene, vinylcyclohexane and 3-methyl-1-butene are especially suitable as the third component.
U.S. Pat. No. 4,551,501 discloses a crystalline propylene polymer composition having excellent transparency and rigidity comprising a blend of a crystalline polypropylene with a polymer of a vinylcycloalkane having at least 6 carbon atoms, which composition contains 0.05 weight ppm to 10,000 weight ppm of the vinylcycloalkane unit. In Comparative Examples 4 to 6 of this patent, a small amount of 3-methyl-1-butene is homopolymerized in the presence of a titanium trichloride catalyst and diethylaluminum and then a large amount of propylene is homopolymerized to form a propylene copolymer containing 0.3 to 2 g, per gram of the titanium trichloride catalyst, of poly(3-methyl-1-butene), and then the copolymer is mixed with a propylene homopolymer to prepare a composition containing 2 to 200 weight ppm of poly(3-methyl-1-butene). It is seen from Table 3 of this patent that the resulting composition is superior in total light transmittance, but inferior in haze value, light scattering index (LSI) and gloss, to the composition prepared in Example 7 of the patent by a similar method and containing 7 ppm of poly(vinylcyclohexane).